This invention relates to a pressure control device particularly adapted for hydrostatic steering systems of motor vehicles.
A pressure control system is disclosed in German patent No. AS 1151446 wherein an annular control passage in a radially outer valve element of a control valve assembly receives the discharge of a pressure medium from the exhausting chamber of a servomotor to conduct the pressure medium to a reservoir tank through a radially inner space enclosed by the valve elements. In such control valve assembly, a low pressure zone exists in the aforesaid annular control passage of the radially outer valve element located between two additional end passages in the outer valve element. The two additional end passages are respectively connected to the outlet end of a metering pump connected to the hand operated steering wheel and the pressurized side of the servomotor, and are hydraulically interconnected through longitudinal grooves in a radially inner valve element when the two valve elements are properly adjusted relative to each other. The flow path so established conducts the pressure medium under a high pressure to the pressurized side of the servomotor. Inasmuch as the low pressure passage in the outer valve element is enclosed between two high pressure end passages in accordance with the foregoing arrangement, a predetermined pressure balance will develop between the aforementioned control passages affected by unavoidable leakage from the two end passages to the intermediate low pressure control passage. Such unavoidable leakage originates from locations in the pressure control system at which any pressure loss directly results in faulty adjustment by the servomotor. Specifically, the loss of pressure medium from the system depletes the supply of pressure medium to the pressurized side of the servomotor from the metering pump. Such decrease in the flow of metered pressure medium to the servomotor results in a corresponding reduction in servomotor adjustment of the steering gear. Further, one of the end passages in the radially outer valve element aforementioned, conducting the pressure medium to the metering pump for metered supply of pressure medium to the pressurized side of the servomotor, is located a relatively short distance from the externally exposed side of the valve elements. Since the internal space of the control valve assembly is connected to the reservoir tank and therefore has the lowest pressure of the system, an unavoidable loss of pressure medium occurring at the externally exposed valve surface results in an additional loss of pressure medium from the internal space of the control valve assembly and a loss of pressure medium from the exhausting side of the servomotor. All of the foregoing losses of pressure medium contributes to a considerable amount of imprecise servomotor adjustment which may be traced back to the unavoidable leakage aforementioned.
It is therefore an important object of the present invention to reduce and substantially eliminate the adverse influence of unavoidable leakage on the operation of the servomotor in a hydrostatic steering system of the type aforementioned.